Greater Khorasan in time processes (pass on the history and geography of greater khorasan)

The greater Khorasan is a vast ancient region. It is an area with a great historical role in Iran, from ancient time. From the time that Aryan came to the top northern part of it and created the first Aryan settler of this part of the world and after a period of time that they came to Iran and India and to the time that they pushed out Parthian settlers to Greece and made Iran just as Iranian home to live. The greater Khorasan became a base in creation of Persian new culture and language in early Islamic centuries. The first Persian dynastic after Islam was Khorasanian Tahirides dynastic and following that Khorasanian Samanides made a very vast territory in there. Khorasan was the first arena for the raid and pillage of Turks, Turkmen and Mongols and after Mongols it experienced another fear and became weak step by step. Although it ruled on Iran for a while at the time of Nadir the King who had raised from there, but after that it was divided into several parts and the two Paris and Akhal treaties made it smaller. Khorasan has presented Ferdowsi, Rudaki, Nasir Khusraw, Abu Rayhan, Avicenna and tens and hundreds of famous people to Iran and the world. Khorasan is a very vast region in the history of Iran and the world as well. This study is a descriptive-analytical research and by means of maps, it tries to do a better analysis and shows the changes of this significant base. Paying attention to Geography and History of the greater Khorasan, this study is going to analyze the reasons of its vastness and the trends of its changes.